1. Field of the Invention
The present invention relates to stack up type low capacitance overvoltage protective device, in particular to stack up type low capacitance overvoltage protective device formed on a substrate and having a construction of a double layered electrode with a voltage sensitive material interposed therebetween.
2. Description of the Prior Art
It is a well known fact that an overvoltage will definitely cause a severe damage to an electrical system. To cope with increasing destructive threat to the speedy and more sophisticated present low voltage system, the intensification for protection to a low voltage system having aforesaid trend becomes more and more important.
Following the increase of system frequency, it is important to reduce the inherent capacitance of an overvoltage protective element as low as possible so as to speed up its response time, by this reasons it is the present tendency to reduce the capacitance of the overvoltage protective device as low as possible to meet the requirement, at the same time, to keep its critical break down voltage to an impulse or surge wave in a low value, but to keep its discharge voltage, ie, the IR drop between its terminal at a proper level such that the follower power current can be interrupted thereby potential stress across the equipment is protected to the lowest feasible value.
There are a plurality of materials known already to be relevantly applicable for use as overvoltage protection materials, among them zinc oxide with property of variable resistance is a preferable selection. There was an element disclosed in U.S. Pat. No. 4,726,991 having a structure that an insulation layer with a thickness smaller than several hundreds A (angstroms) covered with a conductive or a semiconductive powder. In addition, there have been disclosed a plurality of analogous voltage sensitive materials made of mixed powders of various conductive, semiconductive or nonconductive materials with a binder, for example, U.S. Pat. Nos. 3,685,026, 3,685,028, 4,977,357, 5,068,634, 5,260,848, 5,294,374, 5,393,596 and 5,807,509. All these materials are usable for the present invention.
A chip type laminated variable resistor is one of the widely used element, but it has an inherent disadvantage of having relatively higher capacitance, although its capacitance may be reduced to as low as approximately 3 pF by adjusting its construction. Yet the aforesaid critical breakdown voltage and clamping voltage are relatively brought up. A specially constructed semiconductor diode is another choice of the overvoltage protective device, but its capacitance is still not able to overcome the threshold value of below 10 pF.
The structure of the low capacitance overvoltage protective device is characterized by that there is a microgap formed between the two electrodes at both ends of the device with a voltage sensitive material filled therein. Therefore, obtaining a very low capacitance below 1 pF by the fact that the area of the electrode is so small. There are two already known structures of such device: The first one disclosed by U.S. Pat. Nos. 6,023,028 and 5,974,661 formed by common printed circuit board (PCB) fabrication process is shown in FIG. 1 wherein it is composed of a substrate 100 made of an insulation sheet material; a right and a left separated electrode layers 112 and 111 generally formed of copper foil on the insulated substrate 100; a voltage sensitive material layer 120 formed on the right and left electrode layers 112,111 and therebetween; and a main protecting layer 130 covering the electrode layers 111,112 and on the material layer 120. The second one disclosed by U.S. Pat. No. 6,013,358 is formed by thick film printing process is shown in FIG. 2, wherein it is composed of a ceramic substrate 200; a buffer glass material layer 220 divided with minor gaps by cutting formed on the substrate 200; a right and a left electrode layers 232, 231 formed by printing process on the glass material layer 220, these two electrode layers 232, 231 being separating and covering partially the glass material layer 220; a voltage sensitive material layer 240 formed on the electrode layers 232, 231 and extending into the buffer glass material layer 220; and a protecting layed 250 formed on the uppermost part of the whole construction. This overvoltage protective device formed with thick film printing process must employ a special cutting microgaps process to form its essential structure. Although these two devices fabricated with aforementioned process can easily attain a low capacitance property for its main structure, yet there are following inherent shortcomings concerning their fabrication processes:
1. In the PCB process, there is an inherent limitation that the gap distance can not be formed as small as desireously possible to obtain favorably low critical break down voltage and discharge voltage.
2. The stack up structure can be relatively easily formed with thick film printing process incorporated with special cutting process for forming the microgaps than with the former PCB process, but special preparation of extra tools is needed to carry out the cutting process which results in exorbitantly high production cost. Besides, the burrs inadvertently produced on the cutting surfaces between the electrodes may result in a point discharge therebetween and lower the yield rate of production. If slows down the cutting process for evading such an unexpected result, the production cost further rises up.
Aiming at the above depicted problems, the present invention is to propose a newly developed construction and fabrication process for a stack up type low capacitance overvoltage protective device capable of solving those problems which the conventional techniques encounter.
It is an object of the present invention to provide a stack up type low capacitance overvoltage protective device whose low capacitance character can be attained through controlling surface area of printed stack up elements and thickness of a voltage sensitive material layer, the capacitance can be reduced to as low as below 1 pF.
It is another object of the present invention that the thickness of the voltage sensitive material layer can be adjusted by the printing tools and printing parameters so as to keep both the critical breakdown voltage and the clamping voltage of the overvoltage protective device at a reasonably low level.
It is a further object of the present invention that this overvoltage protective device can be fabricated by thick film process so that mass production can be expected with a low production cost by utilizing commonly equipment. Also, it is expected to eliminate point discharge phenomenon by forming surfaces of two adjacent electrodes as smooth as possible.
To achieve these and other objects of the present invention, the stack up type low capacitance overvoltage protective device is composed of a ceramic substrate, a separated conductive lower electrode layer and a conductive upper electrode lead wire layer both formed by printing process on the substrate; a voltage sensitive material layer stacked up on the conductive lower electrode layer by printing process; and a conductive upper electrode layer stacked up on both the conductive upper electrode lead wire layer and the voltage sensitive material layer by printing process. In this version, the construction of a main structure for the present invention is completed, afterward a protective layer is enclosed over this main structure.
The objects, advantages and features of the present invention will become more readily appreciated and understood from a consideration of the following detailed description of preferred embodiments when taken together with the accompanying drawings appended below.